1. Field of the Invention
This invention relates to an image sensing apparatus having an optical low-pass filter and, more particularly, to an image sensing apparatus having an optical low-pass filter suited to two-dimensionally and dispersively (discretely) process an image as in the video camera, electronic still camera, or a like using the color solid-state image sensor.
2. Description of the Related Art
The image sensing apparatus using the solid-state image sensor of discrete picture element structure generally carries out spatial sampling of an optically formed image when the output image is obtained.
FIG. 3 shows the structure of the color filter of the stripe shape for obtaining the color signals in the conventional single-plate type color video camera. The stripes of the color filter are in registry with the respective individual cells of the solid-state image sensor. By this, the separation to three primary colors R, G and B is performed.
As the problem of the case of dispersively sampling the image, mention may be made of the aliasing error occurring when an object to be photographed contains higher frequency components than the spatial frequency of sampling. This error appears in the form of moire and spurious color in the taken image, becoming a cause of distorting the image. Particularly with the single-plate camera, because the sampling pitch is coarse as the horizontal spatial sampling is done once in every three picture elements, color moire or the like is apt to take place.
The processing system that follows the image sensor is designed on the premise that a certain limitation is laid on the band of spatial frequencies of the image. For this reason, it has been the common practice in the art to introduce an optical low-pass filter into the image sensing system so that of the spatial frequencies the object has, the high frequency band is limited and matched to the system that follows the image sensor. As the optical filter, the one which utilizes double refraction, for example, the crystal plate, has been used in many systems.
For example, in the single-plate color video camera apparatus as shown in FIG. 3, the optical system is so constructed that a light beam, when passing through one crystal plate, is split by double refraction into two parts which arrive at the image sensor in displaced relation by 1.5 picture elements in the horizontal line. That the two images of one and the same object superimpose one upon another in offset relation by 1.5 picture elements in the horizontal direction means that the horizontal frequency characteristic of this crystal low-pass filter becomes of the cosine type shown in FIG. 4. On assumption that the pitch of one picture element is d, if the sine wave chart has a pitch of 3d, the contrast becomes zero because the displacement by 1.5 picture elements brings the crest and valley just into coincidence with each other.
From the reason that the sampling frequency fs a picture element is fs=1/d, this crystal low-pass filter has its characteristic take "0" at fs/3, as shown in FIG. 4. In the case of the single-plate color solid-state image sensor, one out of three picture elements is sampled. So, it is from here that the aliasing of color moire originates. Hence, such a filter has a great effect of suppressing the production of moire.
Meanwhile, a technique wherein all the picture elements of the solid-state image sensor are arrayed in the offset sampling form and each picture element of this array is in registry with one of the R, G and B zones of the three-color filter has recently been developed and attention is being paid on it.
FIG. 5 shows an example of the conventional color solid-state image sensor of the offset sampling structure.
In this case, however, because the array of the zones of the color filter is not as simple as with the stripe type, or one-dimensional, but two-dimensional, a problem has arisen that the simple idea of using the conventional crystal low-pass filter as the means for preventing color moire from occurring does not provide the desired effect.
As a patent application which has considered the two-dimensional aspect of the spatial frequency, mention may be made of Japanese Patent Publication No. Sho 57-15369.